Matrix is a communication protocol, just like SMTP (used by email servers) and XMPP (the chat protocol once used by platforms like Facebook). Matrix, however, is not designed to replace these protocols, but rather, to work with them. With a single Matrix account, you can message people on most common platforms. To learn more about Matrix, look at the link above. You can get an account in about 5 seconds, using the Element client.
Matrix works. Matrix is easy. Matrix is free.
Above all else, nobody owns your Matrix account. Facebook could arbitrarily ban you if you violate their interpretation of their rules. They don't have any legal obligation to provide their service to you. You can always have a Matrix account, just like you can always have an email account.
INFORMATION WE COLLECT
When you interact with us through the Services, we may collect information from you, as further described below:
Information You Provide:
We collect information from you when you voluntarily provide such information, such as when you register for access to the Services or use certain Services. Information we collect may include but not be limited to username, email address, and any messages, images, transient VOIP data (to enable communication delivery only) or other content you send via the chat feature.
With Matrix, nobody is collecting data on your messages (except the recipients). Not only can you set up your own Matrix server—a real server, not like a Discord "server" that runs on the company's server—but clients like Element have end-to-end encryption built-in. Flip on a switch, and your messages will be secure. This is why the French Government uses Matrix.
One may argue that Matrix lags behind apps like Slack and Discord. However, we must first note that Matrix is a communication protocol, not an application. Even though the Matrix project was started in 2014, the most popular client—Element—was first released in 2016. Element is free, open source, and made without the benefit of billions in capital. All things considered, Element.io has accomplished a lot already and improves more each day. (At the time of writing, April 2020, Element's most recent stable release was in January 2020, while Discord's most recent stable release was in January 2019.)